1. Field of the Invention
This invention relates to channel allocation in a radio telephone system having base stations with which mobile radios are in radio communication.
2. Description of the Related Art
The invention addresses a limitation in conventional channel allocation whereby radio channels cannot be re-used by adjacent base stations due to the interfering radio coverage zones, even when mobile radios are not operating in those zones. There are radio systems where a proportion of the mobile radios are always located in such isolated areas that the channels used by them can be re-used in neighbouring areas. For instance, at in an underground railway system (where radio base stations are located at each railway station) there can be such radio isolation at the railway stations. However, this isolation does not exist in tunnels interconnecting the railway stations because in the tunnels the radiating cables of the two adjacent base stations meet. If it is necessary to allocate a channel for use in a tunnel it is important that this channel is not re-used on the adjacent cable since this would cause interference and malfunction of the mobile radio roaming technique. Therefore, the use of the channel for a call in a tunnel would normally preclude its re-use at the next railway station even though the mobile radio user may only require radio coverage within the railway station area. This is wasteful of radio channels and the aim of the invention is to improve the efficiency of channel allocation.